1. Field of the Invention
The present invention relates to lasers, and more particularly to passively mode-locked solid state lasers designed to operate at repetition rates exceeding 1 GHz.
2. Description of Related Art
Solid-state lasers are known in the art. Their laser gain media are dopant ions incorporated in dilute concentrations in solid hosts. The laser gain medium can be optically excited to emit electromagnetic radiation by impinging a pumping beam on the laser gain medium.
High-repetition-rate lasers are desirable for a number of applications, such as for use as seed sources for driving radio-frequency photocathodes. These RF photocathodes are then used to inject high-energy electron bunches into a linear accelerator. It is often desirable to have the laser repetition rate operating at the drive frequency of the linear accelerator, which is typically at 2.8 GHz or higher. It is also possible to use high-repetition-rate lasers synchronized to the drive frequency of the accelerator in diagnostic tools or in optical-electron interactions after the electrons are fully accelerated.
Other possible applications of high-repetition-rate lasers are in the area of telecommunications, photonic switching, and optoelectronic testing. As networks and electronic components continue to increase in terms of bandwidth and clock frequency, optical pulsed laser sources become more important for driving, sensing, and testing of these components. One example of this application for optical clocking of integrated circuits is disclosed in U.S. Pat. No. 5,812,708, V. R. Rao, “Method and apparatus for distributing an optical clock in an integrated circuit”.
Mode locking is a special operation regime of lasers where an intracavity modulation (amplitude or phase modulator) forces all of the laser modes to operate at a constant phase, i.e., phase-locked or “mode-locked”, so that the temporal shape of the laser output forms a continuously repeating train of short (typically in the range of picoseconds or femtoseconds) optical pulses. The repetition rate of this pulse train is set by the inverse of the laser round-trip time, or equivalently by the free spectral range of the laser, frep=c/2L where c is the speed of light and L is the cavity length for a standing wave cavity. This repetition rate frep is termed the fundamental repetition rate of the laser cavity, since this corresponds to only one laser pulse circulating in the cavity per round trip. The repetition rate can be scaled by integer multiples N of the fundamental repetition rate under certain conditions, and this is called harmonic mode locking. In this case, there are multiple laser pulses circulating in the cavity per round trip.
The minimum possible pulsewidth of the laser is nominally set by the linewidth of the laser transition, following approximately the condition that tmin≧0.44/Δf where Δf is the linewidth of the laser transition. For typical laser materials such as Nd:YAG or Nd:vanadate (Nd:YVO4), the laser linewidth can support pulses to less than 10 ps. For broader-bandwidth materials such as Nd:glass, Cr:YAG or other Cr-doped garnets, semiconductor materials (in optically pumped surface-emitting semoconductor lasers) or Ti:sapphire, pulsewidths to below 100 fs and even below 10 fs can be generated.
Mode-locked lasers are well known in the state of the art, having been first described in the 1960's (see H. W. Mocker et al., “Mode competition and self-locking effects in a Q-switched ruby laser,” Applied Physics Letters, vol. 7, pp. 270–273, 1965). Passive mode locking using a saturable absorber was discovered almost immediately thereafter. Most mode-locked lasers have used active modulators, where the term “active” means that a source of power such as a radio-frequency signal or another electronic signal must be periodically applied to the modulator. Typical active modulators are acousto-optical modulators (AOMs, Bragg cells) or electro-optical modulators (EOMS, Pockels cells). Active modulators can modulate the amplitude (AOMs or EOMs) or the phase (EOMs) of the optical signal to achieve mode locking.
Active mode-lockers have the disadvantages of cost and complexity. A typical device requires a precision electro-optical component, plus drive electronics that typically consists of high-power, high-stability RF-signal (for AOMS) or high-voltage (for EOMS) components. Additionally, feedback electronics may be required to stabilize either the drive signal for the modulator and/or the laser cavity length to achieve the necessary stability from the system (cf. U.S. Pat. No. 4,025,875, Fletcher et al., “Length controlled stabilized mode-lock Nd:YAG laser”, and Lightwave Electronics, Series 131 data sheet, March 1994).
Active mode locking has been available in commercial lamp-pumped laser systems and more recently in diode-pumped laser systems at repetition rates typically of 100 MHz and extending up to 250 MHz. Research on active mode locking has been done on higher repetition rates, achieving repetition rates of approximately 2 GHz (see K. J. Weingarten et al., “Two gigahertz repetition rate, diode-pumped, mode-locked Nd:YLF laser”, Optics Letters, vol. 15, pp. 962–964, 1990), 5 GHz (P. A. Schulz et al., “5-GHz mode locking of a Nd:YLF laser”, Optics Letters, vol. 16, pp. 1502–1504, 1991), 20 GHz (A. A. Godil et al., “Harmonic mode locking of a Nd:BEL laser using a 20-GHz dielectric resonator/optical modulator”, Optics Letters, vol. 16, pp. 1765–1767, 1991), and more recently 40 GHz (A. J. C. Viera et. al., “Microchip laser for microwave and millimeter-wave generation”, IEEE MTT-S IMOC '97 Proceedings). In all cases the systems required an active modulator driven by a stable RF source and an RF amplifier. The highest repetition rates at 40 GHz were achieved with “harmonic” mode locking (see M. F. Becker et al., “Harmonic mode locking of the Nd:YAG laser”, IEEE Journal of Quantum Electronics, vol. QE-8, pp. 687–693, 1972), where the modulator is driven at some integer multiple of the fundamental laser repetition rate. This is an additional source of complexity and instability in the laser system. In general we wish to avoid harmonic mode locking if possible.
It is also possible to generate high repetition rates using other laser medium such as rare-earth-doped fiber lasers, and semiconductor lasers. Repetition rates of >10 GHz have been demonstrated in semiconductor quantum well lasers (see U.S. Pat. No. 5,040,183, Chen et al., “Apparatus comprising optical pulse-generating means”), achieving pulse repetition rates even above 100 GHz. However, their approach appears to be limited in terms of average power. Fiber lasers have also been demonstrated to high repetition rates using active or harmonic passive mode locking (see U.S. Pat. No. 5,414,725, Fermann et al., “Harmonic partitioning of a passively mode-locked laser”, and S. V. Chernikov et al., “Duration-tunable 0.2–20 ps 10-GHz source of transform-limited optical pulse based on an eletroabsorption modulator”, Optics Letters, vol. 20, pp. 2399–2401, 1995).
Passive mode locking at the fundamental repetition rate, on the other hand, is a much simpler, robust, and lower-cost approach to generating mode-locked pulses. Passive mode locking is also well established in the state of the art (see A. J. DeMaria et al., “Self mode locking of lasers with saturable absorbers”, Applied Physics Letters, vol. 8, pp, 174–176, 1966). The most significant developments in passive mode locking in the recent years have been Kerr-Lens Mode locking (KLM) (U.S. Pat. No. 5,163,059, Negus et al., “Mode-locked laser using non-linear self-focusing element”) for generation of femtosecond pulses from Ti:sapphire and other femtosecond laser systems, and the semiconductor saturable absorber mirror (SESAM) device for generating picosecond and femtosecond pulses in a wide number of solid-state lasers (see U. Keller et al., “Semiconductor saturable absorber mirrors (SESAMS) for femtosecond to nanosecond pulse generation in solid-state lasers,” Journal of Selected Topics in Quantum Electronics (JSTQE), vol. 2, no. 3, pp. 435–453, 1996). Passive mode locking relies on a saturable absorber mechanism, which produces either decreasing loss with increasing optical intensity, or similarly an increase gain with increasing optical intensity. When the saturable absorber parameters are correctly adjusted for the laser system, the optical intensity in the laser cavity is enhanced such that a mode-locked pulse train builds up over a time-period corresponding to a given number of round-trips in the laser cavity.
Most passively mode-locked lasers have been operated at repetition rates of approximately 100 MHz, corresponding to a cavity length of approximately 1.5 m. This cavity length is appropriate for many applications (such as seeding a regenerative laser amplifier) and is also convenient for building laboratory-scale lasers. Work has been done to achieve higher repetition rates, which could be important for telecommunications and optical clocking applications (see U.S. Pat. No. 4,930,131, Sizer, “Source of high repetition rate, high power optical pulses”, U.S. Pat. No. 5,274,659, Harvey, et. al., “Harmonically mode-locked laser”, U.S. Pat. No. 5,007,059, Keller et al., “Nonlinear external cavity mode-locked laser”; B. E. Bouma et al., “Compact Kerr-lens mode-locked resonators”, Optics Letters, vol. 21, 1996, pp. 134–136; and B. C. Collings et al, “True fundamental solitons in a passively mode-locked short-cavity Cr4+:YAG laser”, Optics Letters, vol. 22, pp. 1098–2000, 1997).
However, passive mode locking in solid-state lasers has not been readily achieved at fundamental repetition rates beyond 1 GHz. There are a number of reasons for this limitation. First, for a given average power, the pulse energy and, thus, the peak power in a pulse will decrease as the laser repetition rate increases (given that the pulsewidth also stays constant). For laser relying on peak-power induced nonlinearities to achieve passive mode locking (i.e., lasers using KLM) it becomes increasingly difficult to mode-lock at higher repetition rates. In addition, the cavity size decreases in length inversely proportional to the repetition rate, and it becomes more difficult to adequately provide dispersion compensation. As noted, solid-state lasers using KLM have not been reported substantially beyond repetition rates of 1 GHz (see B. E. Bouma et al., “Compact Kerr-lens mode-locked resonators”, Optics Letters, vol. 21, 1996, pp. 134–136, and U.S. Pat. No. 5,553,093 Ramaswamy et. al., “Dispersion-compensated laser using prismatic end elements”).
For passively mode-locked lasers using SESAMs for mode locking, the limitation on repetition rate is the onset of Q-switching instabilities (see U. Keller et al., “Semiconductor saturable absorber mirrors (SESAMS) for femtosecond to nanosecond pulse generation in solid-state lasers,” Journal of Selected Topics in Quantum Electronics (JSTQE), vol. 2, no. 3, pp. 435–453, 1996, and U. Keller, “Ultrafast all-solid-state laser technology”, Applied Physics. B, vol. 58, pp. 347–363, 1994). This has also limited the laser repetition rate to the range of several hundred megahertz typically. Using the technique of coupled cavity mode locking (RPM), a repetition rate of 1 GHz was demonstrated (see U. Keller, “Diode-pumped, high repetition rate, resonant passive mode-locked Nd:YLF laser”, Proceedings on Advanced Solid-State Lasers, vol. 13, pp. 94–97, 1992). However, this is a much more complicated laser due to the additional laser cavity which has to be carefully aligned with the main laser cavity.
It would be advantageous to achieve repetition rates greater than 1 GHz for many applications such as synchronization with linear particle accelerators (which typically operate at 3 GHz or higher), use in high-speed telecommunication networks as optical pulse sources, and optical clocking of circuits and system in the gigahertz range. These lasers may also find applications in measurement applications such as precision ranging, optical testing of photodetectors and other optically triggered components, and electro-optical test methods on electronics and integrated circuits.
Since an interesting field of application for high-repetition-rate lasers is optical telecommunication, it is also desirable to operate such lasers at wavelengths around 1.3 μm or 1.5 μm, which are most frequently used for signal transmission through glass fibers. Some solid-state gain materials are available for these wavelength domains, e.g., Nd:YVO4 for 1.3 μm or Cr:YAG for 1.3–1.5 μm. A Cr:YAG laser operating at 1.5 μm has been demonstrated (R. Mellish, S. V. Chernikov, P. M. W. French, and J. R. Taylor, “All-solid-state high repetition rate modelocked Cr4+:YAG laser”, Electron. Lett. 34 (6), 552 (1998)) with up to 1 GHz repetition rate. Other Cr-doped garnets also emit in these wavelength domains (cf. S. Kück, K. Petermann, U. Pohlmann, and G. Huber, “Near-infrared emission of Cr4+-doped garnets: lifetimes, quantum efficiencies, and emission cross sections”, Phys. Rev. B 51 (24), 51 (1995)). Using these similar materials with a laser design according to the invention, significantly higher repetition rates should be achievable.